Generally, a cooling system for an engine in a vehicle allows the engine to maintain a normal operating temperature under all running conditions and all speed ranges. This is to prevent thermal damage to the cylinder block, cylinder head, piston and other parts in temperatures of approximately 2,500° C. that is generated by the combustion of fuel-air mixture in a combustion chamber.
In a cooling system using the separate cooling method, as described in the prior art, the sub-thermostat is mounted with a temperature-sensing bulb (sealed therein with wax) exposed to an outlet of a cooling water passage at the cylinder head side to modulate the opening and closing in response to the temperature of the cooling water. As a result, the opening and closing occurs in response to the temperature of the cooling water at the cylinder head side rather than the temperature of cooling water at the cylinder block side.
This problem is caused by a phenomenon where leaking cooling water at the cylinder block side cannot stay around a temperature-sensing bulb of the sub-thermostat by flow of the cooling water discharged to a high temperature passage from the cylinder head side when the cooling water at the cylinder block side is discharged to the high temperature passage connected to a radiator via a leaking passage mounted at the sub-thermostat.
As a result, a problem arises where a sub-thermostat, opened by a pressure difference of approximately 0.6 bar between a cylinder block and a cylinder head, is closed when the temperature of cooling water at the cylinder head side suddenly drops while the temperature of the cooling water at the cylinder block side is high as in the case where a vehicle ascends and descends on a steep hill, thereby preventing the cooling water in the cylinder block from flowing, ultimately causing thermal damage to the cylinder block.